Process for continuous cellulose cooking

ABSTRACT

The invention provides an apparatus and method for producing cellulosic pulp. A downwardly moving column of cellulosic material is contacted with a countercurrent flow of steam in a steaming zone. The steaming zone is maintained at an atmospheric pressure and temperature effective to expel air from the material without causing discoloration thereof. The temperature may be from 100* C. to 120* C. The steamed material is preimpregnated in a zone which is sealed from the ambient air. The preimpregnation is effected by contacting the steamed material with cold liquor having a temperature below that of the steamed material and effective to produce a rapid absorption of liquor therein. The cold liquor temperature is in the range of about 20* C. to about 80* C., and pressures of up to 10 gauge pressures can be employed. A condensate material resulting from the steaming treatment is separated from the cellulosic material after it has been subjected to the steaming treatment. The preimpregnated cellulosic material is sluiced through an impregnating zone containing hot cooking liquor at a temperature and superatmospheric pressure sufficient to effect impregnation thereof. The temperature within the impregnating zone may range from 100* C. to 180* C. and increase in the direction of material movement. The impregnated material is then passed from the impregnating zone for subsequent introduction into any desired pulp vapor phase digesting zone. The disclosed apparatus is used to effect the various process steps of the overall treatment.

United States Patent [72] inventors Arne Johan Arthur Asplund Lidlngo; Karl Nicolaus Cederqulst, Nasbypark; Rolf Bertll Reinhall, Lidingo, all of Sweden [21] Appl. No. 857,295

(22] Filed Sept. 5, 1969 Patented Nov. 9, 1971 [73] Assignee Deflbrator Aktiebolag Stockholm, Sweden [32] Priority Apr. 21,1965

[33] Sweden Continuation-impart of application Ser. No. 543,713, Apr. 19, 1966, now abandoned.

[54} PROCESS FOR CONTINUOUS CELLULOSE Primary Examiner-S. Leon Bashore Assistant Examiner-R. H. Tushin Attorney-Munson & Fiddler ABSTRACT: The invention provides an apparatus and method for producing cellulosic pulp. A downwardly moving column of cellulosic material is contacted with a countercurrent flow of steam in a steaming zone. The steaming zone is maintained at an atmospheric pressure and temperature effective to expel air from the material without causing discoloration thereof. The temperature may be from C. to C. The steamed material is preimpregnated in a zone which is sealed from the ambient air. The preimpregnation is effected by contacting the steamed material with cold liquor having a temperature below that of the steamed material and effective to produce a rapid absorption of liquor therein. The cold liquor temperature is in the range of about 20C. to about 80 C., and pressures of up to 10 gauge pressures can be employed. A condensate material resulting from the steaming treatment is separated from the cellulosic material after it has been subjected to the steaming treatment. The preimpregnated cellulosic material is sluiced through an impregnating zone containing hot cooking liquor at a temperature and superatmospheric pressure sufficient to effect impregnation thereof. The temperature within the impregnating zone may range from 100 C. to C. and increase in the direction of material movement. The impregnated material is then passed from the impregnating zone for subsequent introduction into any desired pulp vapor phase digesting zone. The disclosed apparatus is used to effect the various process steps of the overall treatment.

PROCESS FOR CONTINUOUS CELLULOSE COOKING This application is a continuation-in-part of Ser. No. 543,7l3,filed Apr. 19, 1966, now abandoned.

BACKGROUND OF THE INVENTION The process and apparatus for carrying out the disclosed process are based on known method steps. The production of chemical and semichemical sulfite, bisulfite and neutral sulfite pulps by digesting wood impregnated with cooking liquor is well known in the prior art. The prior art process includes the steps of l) saturation of chips with steam, (2) impregnating the steamed chips with cooking liquor, (3] sulfiting the impregnated chips in cooking liquor and (4) hydrolysis and digestion of the sulfited chips in vapor phase.

More particularly, the process and apparatus of this disclosure the production of cellulose fiber pulps of finely divided ligno-cellulosic material. The ligno-cellulosic material is digested with cooking liquor substantially containing sulfite soluble bases eg sodium, ammonium hydrate and magnesium. The cooking liquor used in this process and apparatus has a pH value lying in the range between 3 and 12. The sulfite solutions can thus be constituted of bisulfite with or without free S mixtures of sulfite and bisulfite, pure sulfite or sulfite solutions, buffered with materials such as soda, sodium carbonate or alkali hydrate. The ligno-cellulosic material can be constituted of wood, bagasse, straw and the like which is generally designated in the following disclosure as chips.

PURPOSE OF THE INVENTION The primary object of this invention is to provide a process to effectuate various method steps continuously in a completely closed system.

Another object of this invention is to provide the technical and mechanical means required to conduct the process disclosed herein.

SUMMARY OF THE INVENTION It has been found through experimentation that a suitable preimpregnation ofthe chips with cooking liquor is achieved if the chips are first saturated with steam. The steam zone is maintained at a temperature not exceeding 120 C. A specific embodiment of this invention maintains the said temperature at about 100 C. After preimpregnation, the hot chips are immersed and cooled in cold cooking liquor which is standing under atmospheric pressure. The contact between the hot chips and the cold cooking liquor is effected in a manner to prevent the hot chips from coming into contact with the ambient air. In another embodiment of this invention, the steam treated hot chips are rapidly infused and cooled with cold cooking liquor and at the same time subjected to a hydraulic cooking liquor pressure of from 2-10 gauge pressures. The cold cooking liquor is maintained at a temperature of less than about 80 C. It has been found that the results of the process are much improved as the temperature of the cold cooking liquor is decreased. A further temperature range for the cold cooking liquor is below about 50 C. In production practice, the temperature of the cold cooking liquor is maintained as near room temperature as possible in a range from about 20 C. to 40 C. The temperature of the cold cooking liquor is selected so that it will be effective to produce a rapid absorption of liquid in the material that has been treated in the steaming zone.

The preimpregnated chips are then introduced into an impregnating zone having hot cooking liquor standing under pressure and a high temperature without the risk of forming dark cores with consequent impairment of color of the wood pulp or reduction in the strength of the wood pulp. The temperature and pressure are maintained in the impregnation zone to ensure a rapid and thorough impregnation and uniform concentration of the impregnating agent into the chips. The temperature ofthe impregnating zone may be from about 120 C. to 180 C. A more limited range of the temperature in the impregnating zone is from about 100 C. to [70C.

It has been found advantageous to correlate the temperature of the hot cooking liquor and the duration of stay of the chips in the liquor in a manner to ensure the the chips are sulfited. The degree of sulfitation is dependent on the composition of the cooking liquor, of the type of cellulosic material, and the yield desired to obtain. The sulfiting process shall be carried out in such a manner that only an insignificant portion of the sulfited material is hydrolized and released. Optimal results with respect to the quality and strength properties of the pulp in subsequent digestion of the chips in a vapor phase are attained under such working conditions.

The chips impregnated according to the disclosed process and which. are sulfitized to a lesser or greater degree, can be heated directly in a saturated steam atmosphere of 200 C. without jeopardizing the strength of the wood pulp. The hydrolysis and digesting reactions are very rapid under these conditions. If a sodium sulfite cooking liquor within the lower pH range is used, it is possible to produce from chips such as spruce, a chemical wood pulp with a yield of from 32-58 percent in 30 minutes. Here the lower pH range is approximately 3 and may be used e.g. for impregnating and sulfiting in sufficient Na O of base, viz about 6-10 percent, is present.

If the digestion of the cellulosic material is carried out at C. with the same type of cooking liquor at a pH of about 4 and a cooking time of approximately 1 hour, the yield lies about 70 percent.

Digestion in vapor phase at temperatures lower than [50 C. are of small interest since the digestion time is unproportionally long and the properties of the pulp are not improved. By introducing the preimpregnated, cooled chips into the hot cooking liquor, it is possible to heat the chips for a shorter period of' time in a steam atmosphere before they are fed into the cooking liquor. This combination of process steps may be accomplished without jeopardizing the properties of the pulp. The temperature of the steam should not be higher than that which has been previously stated for the cooking liquid and thus suitably should be kept between 120 and 180 C.

The apparatus used in effecting the process ofthe instant invention comprises a vertical vessel wherein a large temperature gradient is established between the top and the bottom so that the chips which are charged into the vertical vessel are subjected This a continuously rising temperature on their passage therethrough. The cooking liquor in the vertical vessel is difficult under a hydraulic pressure. The preimpregnated chips are fed into this cooking liquor and thereby subjected to the temperature gradient within the said vessel. The bottom temperature of the vessel can rise for example to about l70 C. and the temperature along the top of the vessel may lie at from about l00C. to 120C.

Another feature of the structural apparatus used in conjunc tion with this invention is to connect a fibrator to the bottom of the digester. This structural assembly makes it possible to defibrate the wood pulp under pressure before it is sluiced out. This type of arrangement is especially useful when manufacturing semichemical pulps and other fiber pulps that are difficult to defibrate. Defibrating of these pulps can be carried out at digestion temperatures or at lower temperatures. When using lower temperatures, it is more simple to cool the wood pulp by adding sulfite liquid, release liquid or water immediately before the defibrator.

Any cooling liquid should be added in such a manner that it does not enter the digester. This action will prevent unnecessary increase in steam consumption during the digestion period.

The material may be sluiced out without being defibrated. This can be effected with or without cooling the fiber pulp. If cooling must be effected, the cooling, liquid is added in the sluice without being first fed into the digester. The fiber pulp can also be continuously blown out through a nozzle, blow valve or a throttle valve with or without addition of cooling liquor.

BRIEF DESCRIPTION OF DRAWINGS Other objects of this invention will appear in the following description and appended claims, reference being made to the accompanying drawings forming a part of the specification wherein like reference characters designate corresponding parts in the several views.

FIG. 1 is a partial cross-sectional view of an apparatus made in accordance with this invention to carry out the process of the invention,

FIG. 2 is a partial cross-sectional view of a portion of the apparatus showing the conveyance of material from the steaming zone to the impregnating zone.

FIG. 3 is another embodiment of an apparatus accomplishing the results of the apparatus of FIG. 2,

FIG. 4 is a partial cross-sectional view of a further embodiment of an apparatus to effect the process of this invention,

FIG. 5 is another embodiment of the conveyance mechanism from the sulfiting vessel to the digester,

FIG. 6 is a still further embodiment of another conveyance system as illustrated in FIG. 5, and

FIG. 7 is a partial cross-sectional view of an apparatus used for emptying the digester.

DESCRIPTION OF SPECIFIC EMBODIMENTS More specifically, the apparatus assembly as seen in FIG. 1 includes three separate units; namely, a steaming vessel 2, a sulfiting vessel 23, and a digester 34. These vessels provide a steaming zone, an impregnating zone, and a digesting zone, respectively. The units are interconnected by transfer valve devices 7 and 29. These transfer valve devices 7 and 29 are known in the art and their specific operation does not form a part of this invention. A further transfer valve sluice device 38 is used on the cellulosic material upon emptying the digester 34. Wood chips are fed into the steaming zone formed by the steaming vessel 2 by way of the conveyor 1. A predetermined levelof the wood chips is maintained in the vessel 2 while a countercurrent flow of steam is supplied upwardly within the vessel 2 through a steam conduit 5. The chips rest on a mobile grid 3 which is used to feed the steamed chips downwardly to the bottom of the steam vessel 2. The steamed chips are subsequently fed downwardly into the transfer valve device 7 by a rotating impeller 4 mounted in the bottom of the vessel 2. The apparatus as described hereinabove performs the step of contacting a downwardly moving column of cellulosic material in a steaming zone with a countercurrent fiow of steam. This steam atmosphere is at a pressure and a temperature effective to expel air from the material without causing discoloration thereof. In this embodiment, the pressure is atmospheric pressure and the temperature does not exceed 120 C. Resultant condensate material obtained in the steaming of the cellulosic material departs through a flat screen (not numbered) which is positioned at the bottom of the vessel 2 and is subsequently expelled from the vessel 2 through the conduit 6.

Cold cooking liquid is fed to the transfer valve device 7 through a conduit 9. The steamed material from the vessel 2 is preimpregnated in the zone defined by the transfer valve device 7 which is sealed from the ambient air. The steamed material is contacted with cold liquor having a temperature below that of the steamed material and effective to produce a rapid absorption ofliquor therein. The chips as steamed in the vessel 2 are cooled and subjected to a hydraulic positive pressure by the pump 8. The preimpregnated chips are thereafter fed together with surrounding cooling liquor into the sulfiting vessel 23 which forms the impregnating zone. The sulfiting vessel 23 may be filled completely with cooking liquor standing under hydraulic pressure or, alternatively, may be filled with liquid to a predetermined level and subjected to steam pressure. In either vase, it is apparent from the conditions in the vessel and the accompanying drawings that the sulfiting vessel 23 is a pressure vessel having means for maintaining superatmospheric pressure therein. The transfer valve device 7 includes a rotary feeder shown in dotted lines. As the rotary feeder rotates, it carries cooking liquid or steam from the sulfiting vessel 23 to a vessel I2. The cooking liquid or steam from the sulfiting vessel is blown from the transfer valve device 7 through a conduit 10 which carries the liquid or steam through the cooler 11. Cold cooking liquor is stored in the vessel 12. Sulfur dioxide which may be released and other noncondensable gases are led away through a conduit 13. These gases are washed in a scrubber 14 with cold cooking liquid which is thereafter passed into the vessel 12.

When the sulfiting vessel 23 is completely filled with cooking liquor under hydraulic pressure, a relatively large temperature difierence can be maintained between the top portion and the bottom portion of the vessel 23. That is, the impregnating zone is maintained at a temperature and a superatmospheric pressure sufficient to effect impregnation of the cellulosic material. Under these conditions, the chips are subjected to a continuously rising temperature during the passage downwardly through the cooking liquid. The concentration of cooking liquid is diffused into the chips primarily in the top portion of the vessel 23. Equalization of this diffused concentration is effected with the sulfiting process which is accomplished in the bottom portion of the vessel 23.

When the sulfiting vessel 23 is first filled to a predetermined level by the preimpregnated chips, they are subjected to an atmosphere of steam before they are immersed in the cooking liquor. Such a procedure has no unfavorable influence on the wood pulp provided that the preimpregnating process has been carried out as described hereinbefore. The impregnating zone in the sulfiting vessel 23 has an introduction section and an end section. The cooking liquor in the impregnating zone is recirculated from the end section to the introduction section thereof to reduce the temperature gradient existing in the impregnating zone between the two sections.

Circulation of liquid in the sulfiting vessel 23 is effected with a pump 24 and conduits 25 and 26. The conduits 25 and 26 in this embodiment constitute means for circulating hot cooking liquor from the bottom portion to the top portion of the impregnating vessel. The introduction section is in the top portion of the vessel 23 and the end section of the impregnating zone is in the bottom portion of the vessel 23. Heating of the liquid circulated to the sulfiting vessel 23 is accomplished with the preheater 27. The circulation of the heated liquor is the manner in which the temperature gradient is controlled within the sulfiting vessel 23.

The sulfited chips are fed from the sulfiting vessel 23 through the transfer valve device 29 together with an amount of liquid surrounding the chips. The liquid is separated from the chips before the chips are allowed to fall downwardly into the digester 34. The separated cooking liquid departs through a conduit 30 to a vessel 31. This cooking liquid is subsequently recirculated through the impregnating vessel 23 by way of the pump 32 and conduit 33.

A constant layer of chips is maintained in the digester 34. The duration of stay is controlled by the level and discharge velocity from the bottom of the digester 34. The chips rest on a mobile grid 35 which feeds the chips downwardly and are subsequently conveyed to the sluicing device 38'by the rotating feeder 37 which is located in the bottom of the digester 34. Cold sulfite liquid, released liquid or water may, if necessary, be added to the sluicing device 38 from a container 39. The liquid is pumped into the sluicing device 38 by means of a pump 40 and a conduit 41 so that the wood pulp is cooled be fore being sluiced out.

A transfer valve device 29 is located between the sulfiting vessel 23 and the digester 34. A rotary feeder shown in dotted lines is rotated and thereby carries steam and gases from the digester 34. The steam and gases are blown offthrough a conduit 17 and cooled in a scrubber 18 with circulating condensate through a circulation pump 19, a conduit 20 and a cooler 21. Surplus condensation departs through a conduit 22 and sulfur dioxide which may be released and noncondensable gases are led away to the scrubber 14 through a lead way (not numbered).

The mechanisms for transporting chips to the transfer valve devices 7, 29 and 38 from the steaming vessel 2, sulfting vessel 23 and digester 34 respectively, are only examples of specific embodiments. There are other known devices which can be used in the new combination of structural elements which form the apparatus of this invention. Although it is more suitable to use the transfer valves of the rotary feeder type, the process of this invention may well be carried out by other known types of apparatus to effect the same advantages thereof.

It may be desirable and suitable to provide for a rapid and effective defibration of pulps when semichemical fiber pulps and other fiber pulps not easily fibrated are produced. To accomplish this result, a defibrator may be directly connected to the cooker in front of the sluicing device 38. As more specifically shown in FIG. 7, the defibrator 44 is connected directly to the bottom of the digester 34. The discharge sluicing device 38 is inserted directly behind the defibrator 44. Thus a defibrating operation is carried out under digester pressure and digester temperature before the fiber pulp is sluiced out. When the pressure drop between the digester 34 and the defibrator 44 is insignificant, the digested chips may be positively fed to the defibrator 44 by means of a screw conveyor 43. It is within the contemplation of this invention that any mechanism such as a blow valve or other suitable type of choke valve may be used in place of the sluicing device to blow the pulp out of the apparatus of this invention.

Two further embodiments of apparatus assemblies used for cooling and impregnating steamed chips are shown in FIGS. 2 and 3. Cold cooking liquor is used to both cool and preimpregnate the steamed chips before they are fed into the sulfiting vessel 23. In each of these embodiments, the sulfiting ves sel 23 is under steam pressure and is filled with cooking liquor to a predetermined level. The steamed hot chips fall downwardly from the steaming vessel 2 straight into the container 45 which can be vertically mounted as shown in FIG. 2. The container 45 may also be horizontally mounted as shown in FIG. 3. When the chips are fed horizontally, it is necessary to convey them by means of a screw conveyor as shown. Cold cooking liquor is pumped from the vessel 12 to either the vertical or horizontal containers 45 by means of the pump 8 through the conduit 9. A constant level of the cooking liquor is maintained in the container 45. Surplus cooking liquor is returned to the vessel 12 through the conduit and the cooler 11. Chips and cooking liquid which are fed downwardly by the rotary feeder 7 are already subjected to hydraulic cooking liquid pressure in the rotary feeder 7 by means of pump 46 which is connected to the feeder 7 by a conduit 47. Steam and gases from the sulfiting vessel 23 are blown off through conduit 48 when they enter the rotary feeder 7. When necessary, steam and gases can be cooledand along with possible noncondensable gaseous matter be led to the scrubber 14 as shown in FIG. I to be washed with the cold cooking liquor.

The apparatus as shown in FIG. 4 may be used for feeding chips into a sulfiting vessel 23 which is completely filled with cooking liquor which is standing under hydraulic pressure. The steamed hot chips are fed through the grid 3 into the steaming vessel 2. The steamed chips are carried by the screw conveyor 4 to the rotary feeder 7. The condensate formed in the steamed material is separated off at the same time through the conduit 6. The chips are cooled and pressure impregnated with the cold cooking liquor within the rotary feeder 7. The cooking liquor is fed under pressure to the feeder 7 from the vessel 12 by pump 8 through the conduit 9. The chips in the rotary feeder 7 are fed into curved tubes 52 together with cooking liquor which surrounds the steamed product within the feeder 7. The curved tubes 52 are connected to the top of the sulfiting vessel 23. A strong liquid flow is maintained in the tubes by circulation of cooking liquid from the sulfiting vessel 23 by means ofa pump 50 and a conduit 51. The circulation of cooking liquid flushes the rotary feeder 7 clean and the chips are carried with the flowing liquid and introduced into the sulfiting vessel 23. The liquid carried by the rotary feeder 7 from the curved tube 52 is blown through the conduit 10 and the cooler 11 to the vessel 12. Possible noncondensable gases and sulfite dioxide are led off through the conduit 13 to the scrubber 14 as shown in FIG. 1 for washing with cold cooking liquor.

Two embodiments of an apparatus for feeding chips from the sulfiting vessel 23 to the digester 34 are shown in FIGS. 5 and 6. These devices separate free cooking liquid from the chips at the same time the chips are being fed into the digester 34. In the embodiment of apparatus as shown in FIG. 5, the cooking liquor is separated from the sulfiting vessel 23 and the digester 34. The impregnated chips are fed together with surrounding liquid through the rotary feeder 29. A screw con veyor 61 carries this material over a screen 62 through which the cooking liquor is drained off. The impregnated chips are then conveyed to a rotary feeder 60 and on into the digester 34. The cooking liquor filtered through the screen 72 is blown to the vessel 31 and subsequently recirculated to the sulfiting vessel 23 by way of the conduit 33 and the pump 32. Steam and gases from the digester 34 are entrained by the rotary feeder 60 and subsequently blown off through the conduit 17. In a further embodiment of this apparatus shown in FIG. 5, the sluicing device 60 can be omitted where condensation of stem from the digester 34 can be permitted in the screw conveyor 61. It is also possible to omit the sluicing device 60 is the pressure and temperature in the digester 34 and sulfiting vessel 23 are of the same magnitude. In the latter case, steam and gases from the digester 34 would be blown off through the rotary feeder 29. Obviously a conduit would have to be supplied in the rotary feeder 29.

Another embodiment of the device for transporting impregnated liquid from the sulfiting vessel 23 to the digester 34 is shown in FIG. 6. The chips are fed together with surrounding cooking liquor from the rotary feeder 29 to an open con tainer 70 which is located within the digester 34. The cooking liquor in the container 70 is separated by means of a screen 7I and blown to the vessel 34 for further transportation to the sulfiting vessel 23. This recirculation of cooking liquor is accomplished through the use of the pump 32 and conduit 33. The container 70 includes rotating impeller 72 mounted therein which causes the chips to fall downwardly into the digester 34.

While the process and apparatus for continuous cooking of cellulosic material has been shown and described in detail. it is obvious that this invention is not to be considered as being limited to the exact form disclosed, and that changes in detail and construction may be made therein within the scope of the invention, without departing from the spirit thereof.

Having thus set forth and disclosed the nature of this invention, what is claimed is:

l. The method of producing cellulosic pulp in a closed system comprising:

a. contacting a downwardly moving column of cellulosic material in a steaming zone with a countercurrent flow of steam at atmospheric pressure and at a temperature not exceeding 120 C. to expel air from the material without causing discoloration thereof,

b. preimpregnating the steamed material in a zone sealed from ambient air by contact with cold liquor having a temperature of between about 20 C. and less than about C. such that the temperature of said cold liquor is below that of the steamed material and effective to produce a rapid absorption ofliquor therein,

c. separating a resultant condensate material from the cellulosic material before the cellulosic material enters the preimpregnating zone,

d. sluicing the preimpregnated cellulosic material through an impregnating zone containing hot cooking liquor at a temperature of from about [00 C. to about I80" C. and a hydraulic pressure sufficient to effect impregnation of the cellulosic material and e. passing the impregnated material from the impregnating zone into a subsequent pulp digesting zone.

2. The method as defined in claim 1 wherein the contact between the cellulosic material and the cold liquor is effected by immersion.

3. The method as defined in claim 1 wherein the impregnating zone has an introduction section and an end section and the cooking liquor in the impregnating zone is recirculated from the end section to the introduction section thereof to reduce the temperature gradient existing in the impregnating zone between said sections.

4. The method as defined in claim 1 wherein the cellulosic material is subjected to a hydraulic pressure during the preimpregnating step.

5. The method as defined in claim 4 wherein the hydraulic pressure is of about 2-10 gauge pressures.

6. The method as defined in claim 1 wherein the cooling liquor is separated from the impregnated material prior to passing into said digestion zone.

7. The method as defined in claim 6 wherein the impregnated material is digested in the digesting zone having an atmosphere of steam at a temperature of about 150 C. to 200 C.

8. The method as defined in claim 7 including the step of passing the digested material from the digesting zone to an evacuation zone where it is cooled to a temperature sufficient to effect further pulp treatment steps.

9. The method as defined in claim 1 wherein the impregnating zone is filled with hot cooking liquor to a predetermined level with an atmosphere of steam thereabove, such that the mixture of said material and said cold cooking liquor entering the impregnating zone is subjected to said atmosphere of steam.

104 The method as defined in claim 1 wherein surplus cold liquor from the preimpregnating of the steamed material is cooled and recirculated to its source for further use in the preimpregnating zone.

11. A continuously operable pulp digesting apparatus for eating cellulosic material comprising a. a steaming vessel in which the cellulosic material to be digested descends in contact with a countercurrent flow of steam,

. a preimpregnating vessel connected to said steaming vessel, said preimpregnating vessel consisting of a transfer valve,

c. means for expelling condensate from the cellulosic material before the cellulosic material enters the preimpregnatin g vessel,

an impregnating pressure vessel below said preimpregnating vessel and connected thereto,

. means for passing cold liquor through said preimpregnating vessel,

means for circulating hot cooling liquor from the bottom portion to the top portion of said impregnating vessel,

a digesting vessel below said impregnating vessel,

. valve means located between said impregnating vessel and said digesting vessel for sluicing impregnated material into the digesting vessel,

. means for evacuating the free cooking liquor through said valve means to return it to said impregnating vessel and means for simultaneously evacuating steam and gases from the digesting vessel.

12. An apparatus as defined in claim ll wherein a curved tube is located between the transfer valve and the said impregnating vessel.

* 1K i I. 

2. The method as defined in claim 1 wherein the contact between the cellulosic material and the cold liquor is effected by immersion.
 3. The method as defined in claim 1 wherein the impregnating zone has an introduction section and an end section and the cooking liquor in the impregnating zone is recirculated frOm the end section to the introduction section thereof to reduce the temperature gradient existing in the impregnating zone between said sections.
 4. The method as defined in claim 1 wherein the cellulosic material is subjected to a hydraulic pressure during the preimpregnating step.
 5. The method as defined in claim 4 wherein the hydraulic pressure is of about 2-10 gauge pressures.
 6. The method as defined in claim 1 wherein the cooking liquor is separated from the impregnated material prior to passing into said digestion zone.
 7. The method as defined in claim 6 wherein the impregnated material is digested in the digesting zone having an atmosphere of steam at a temperature of about 150* C. to 200* C.
 8. The method as defined in claim 7 including the step of passing the digested material from the digesting zone to an evacuation zone where it is cooled to a temperature sufficient to effect further pulp treatment steps.
 9. The method as defined in claim 1 wherein the impregnating zone is filled with hot cooking liquor to a predetermined level with an atmosphere of steam thereabove, such that the mixture of said material and said cold cooking liquor entering the impregnating zone is subjected to said atmosphere of steam.
 10. The method as defined in claim 1 wherein surplus cold liquor from the preimpregnating of the steamed material is cooled and recirculated to its source for further use in the preimpregnating zone.
 11. A continuously operable pulp digesting apparatus for treating cellulosic material comprising a. a steaming vessel in which the cellulosic material to be digested descends in contact with a countercurrent flow of steam, b. a preimpregnating vessel connected to said steaming vessel, said preimpregnating vessel consisting of a transfer valve, c. means for expelling condensate from the cellulosic material before the cellulosic material enters the preimpregnating vessel, d. an impregnating pressure vessel below said preimpregnating vessel and connected thereto, e. means for passing cold liquor through said preimpregnating vessel, f. means for circulating hot cooking liquor from the bottom portion to the top portion of said impregnating vessel, g. a digesting vessel below said impregnating vessel, h. valve means located between said impregnating vessel and said digesting vessel for sluicing impregnated material into the digesting vessel, i. means for evacuating the free cooking liquor through said valve means to return it to said impregnating vessel and j. means for simultaneously evacuating steam and gases from the digesting vessel.
 12. An apparatus as defined in claim 11 wherein a curved tube is located between the transfer valve and the said impregnating vessel. 